This invention concerns a thermostat for controlling heating, ventilation, and air conditioning (HVAC) systems in a manner that conserves expenditure of energy and lessens utility peak-load. The invention also concerns a method for operating HVAC systems under the control of such a thermostat to conserve energy and lessen utility peak-load. The invention is directed particularly to such operation of furnaces of such HVAC systems.
The use of short heating cycles to conserve energy in furnaces is taught by, among others, Phillips et al. U.S. Pat. No. 4,199,023. Carney et al. U.S. Pat. No. 4,725,001 generally reviews the prior art in this field including on/off cycling techniques previously taught.
While the Carney et al. thermostat may provide savings in energy usage, the present inventors believe that its technique has limitations that prevent achievement of energy savings beyond a certain point. More specifically, its technique of manipulation of on-time and off-time intervals until the system runs into the edge of the deadband of the set-point is only indirectly related to the HVAC system parameters and the model characterizing the system. That in turn limits the optimization possible, particularly when system parameters (such as ambient temperature) change. Hence, the Carney et al. thermostat can cause the HVAC system to operate under conditions of heat-exchanger saturation or nonlinearity, which leads to fuel waste. In addition, the cycling system using incrementation or decrementation of on-time and off-time intervals requires a many-cycle period to catch up with changes in temperature on load.
While Carney et al. and Phillips et al. recognize the desirability of short on-time intervals, they (as well as others using increment/decrement cycling methods) fail to provide thermostat systems that consistently avoid partial saturation of the HVAC system's heat exchanger. Rather, their and other prior art thermostats utilize "continuous burn" cycles (also referred to as 100% duty cycles) to reach set-point temperature, when that is deemed necessary to overcome a temperature excursion. They do so despite the fact that such operation leads to full or partial saturation of the heat exchanger with a consequent adverse effect on fuel consumption efficiency.
Similarly, although Karl et al. U.S. Pat. No. 4,938,684 discloses limitation of fuel-on intervals to a given maximum value, the length of the interval thus taught is not related to the saturation characteristics of the heat exchanger of the HVAC system.
Further, total saturation of a heat exchanger is not the only operating region in which the heat exchanger is inefficient. A heat exchanger may not be fully saturated to the point where no heat exchange at all occurs. Nevertheless, to the extent that the length of the fuel-consuming interval is so long that significant nonlinearity occurs, then inefficient fuel utilization occurs because of relatively poorer heat transfer. It is believed that prior art thermostats fail to teach the importance of consistently remaining in a linear operating region of the HVAC system's heat exchanger and avoiding more than minimal partial saturation of the heat exchanger. This issue is discussed in greater detail in the specification of the parent patent application of which this patent application is a continuation-in-part, said specification being incorporated herein by reference.
It is also believed that the prior art does not address the issue of making heat flux from the furnace to the heated space equal heat flux from the heated space to the ambient. In particular, the prior art does not address determination of system parameters characterizing such flux, so as to regulate it at a desired equilibrium. Rather, prior art devices in this art depend on temperature measured at the thermostat to regulate space temperature, on a feedback basis. Thus, when the difference between measured space temperature at the thermostat and a predetermined setpoint temperature exceeds a predetermined threshold, the heating system is actuated (or deactivated).
It is known to use wide "deadbands" for thermostats of such systems, providing hysteresis to counteract such effects as undue system fluctuation or "hunting," which may at times result from slight drafts caused by a person walking past the thermostat. That feature has been considered inconsistent, however, with maintaining a tight temperature regulation. Accordingly, the system described in the patent of which the instant patent application is a continuation-in-part attempted to minimize the width of the deadband. That system and other prior art systems, however, did not consider what effect, if any, that the width of the deadband might have on peak or daily energy consumption.